1. Field of the Invention
The present invention relates to power factor correction in single or polyphase alternating current systems, and, more particularly, to a method and apparatus for effecting near-instantaneous power factor correction in a variable reactive load.
2. Description of the Related Art
Producers and consumers of large amounts of electrical power have long sought to optimize the use of power to avoid energy waste. One way to optimize the use of electrical power is to maximize the power factor associated with a reactive electrical load. A reactive load is one which has inductive or capacitive components. An alternating current power source generally provides a sinusoidal voltage and the load draws a sinusoidal current. In a purely resistive load, the source voltage and source current are in phase and the power factor is equal to one. In an ideal purely reactive load (zero resistance), the source voltage and source current are 90.degree. out of phase, and the power factor is equal to zero. The power factor has a fractional value between zero and one for loads which are a combination of resistive and reactive components. The use of electrical power is optimized when the power factor is equal to one.
The power factor is the ratio of the average power required by the load to the apparent power delivered by the source. The power factor thus indicates the ratio of useable power delivered to the load to the amount of power which must be generated at the source. With a low power factor, substantially more line losses occur relative to the useful power delivered to the load, than with higher power factors approaching unity.
When a load requiring a given average power has a relatively high power factor, the power generator need not supply as much excess power to operate the load properly. When the power factor is low, the generator must supply a relatively large amount of excess power, which is accompanied by higher losses in the power lines. The power factor ranges from zero to one depending on how far out of phase the instantaneous voltage and current are at the load. In equation form, the power factor (PF) is expressed as PF=cos(.crclbar..sub.v -.crclbar..sub.i), where .crclbar..sub.v is the phase angle of the voltage and .crclbar..sub.i is the phase angle of the current at a given time.
A typical industrial load may be a bank of induction motors or other expensive machinery. The load requires a certain amount of average power to operate properly. Usually, an industrial load has an overall reactance which is characterized by resistive and inductive components. The power factor in such a load is said to be lagging, since the current through the load lags behind the voltage applied to the load in such a case. This is termed a lagging power factor. In a capacitive load, the current leads the voltage, resulting in a leading power factor.
The apparent power is the product of the root mean square source voltage and the root mean square source current. The apparent power is normally stated in volt amperes (VA) or kilovolt amperes (kVA) in order to distinguish it from average power, which is measured in watts (W).
A low power factor at the load means that the power company's generators must be capable of delivering more current at constant voltage, and they must also supply power for higher line losses than would be required if the power factor of the load were high. Since high line losses represent energy expended in heat and benefit no one, the power company often will insist that a plant maintain a high power factor, typically 0.90 lagging, and will adjust their rate schedule to penalize users that do not conform to this requirement. However, installations that require large amounts of power have a wide variety of load conditions, which may include loads which vary with time.
For instance, consider a single 25 horsepower walking beam oil pump motor. Such motors are used to drive the "see-saw" type oil pumps commonly seen in many oil fields, which are known as walking beam oil pumps. During one part of the cycle of a walking beam oil pump, a large column of pipe is lifted. The motor consumes a relatively large amount of average power at this point. However, during another portion of the cycle of the walking beam oil pump, the column of pipe is being lowered again. At this point, the motor consumes very little average power. The reactance of the motor changes throughout the cycle as the load varies. Hence, the motor behaves as a variable reactive load, with typical oil pump motors including several pumping cycles per minute.
Variable reactive loads present a problem regarding power factor correction since the power factor varies as the reactance varies. Conventional power factor correction schemes, such as fixed capacitors, suffer from the disadvantages of being large, costly, and effective only for a very narrow range of load variations. During transient conditions such as during motor start-up or during drastic load changes, conventional fixed value power factor correction devices save little or no energy. Furthermore, fixed value power factor correction devices are ineffective whenever the load maintains a reactance value outside the designed correction range.
Other conventional devices have also been proposed to solve the problem of conserving energy losses associated with a variable power factor in a variable reactive load. For example, U.S. Pat. No. 4,387,329 issued to Harlow discloses a Three Phase Power-Factor Control System For A.C. Induction Motors. The disclosed device uses a pulsed power-factor signal to control the on time of the power fed to a first-phase winding of an induction motor, and uses the signal indirectly to control the on time of the power fed to both a second-phase winding and a third-phase winding in the motor. The device discloses a plurality of control valves 14a, 14b, and 14c in FIG. 1 (comprising triacs 48, 50, and 52 shown in FIGS. 3 and 4) inserted in series in each power line of the three phase system. The power factor of the first phase winding is monitored, and a wave pulse power factor signal is used to control the on time of the power in the first, second, and third windings in the motor.
Harlow utilizes a switching device or control valve, such as a triac or silicon controlled rectifier (SCR), inserted in series with the variable load device, which has the disadvantage of causing the variable load device to cease functioning in the event of a failure in the series-connected switching device or control valve. Another disadvantage of using a series-connected switching device is that high levels of electromagnetic interference are injected into the system along with harmonic distortion on the power lines. Harmonic distortion increases the heat dissipation in the power lines, lowering the efficiency of the system and reducing component life-spans and reliability. A series-connected switching device also requires costly and time-consuming installation procedures and precautions. It is believed that all series-connected switching devices suffer the same disadvantages.
U.S. Pat. No. 4,723,104 issued to Rohatyn discloses an Energy Saving System For Larger Three Phase Induction Motors. This device employs series and preboost transformers, and includes variable transformers which are motor driven, some of which are connected in series in the power lines. These transformers are costly, and it is implicit in the disclosure that this scheme is cost effective mostly for loads utilizing very large quantities of power. The reference does not indicate the advantages of using (non-linear) switching devices to provide for power factor correction, but suggests the use of transformers operated continuously in the linear region of their hysteresis curve.
Therefore, it would be desirable to provide an apparatus for controlling a power factor in a variable reactive load, wherein a pulse-controlled reactive circuit is attached to the power lines of an AC motor or other variable reactive load, for correcting the power factor of the variable reactive load by storing the difference of the reactive and resistive energy components at a portion of the power cycle while energy is being returned to the power line by the variable reactive load and by pulsing that stored energy back to the power line when the variable reactive load is storing energy.
Similarly, it would be desirable to provide an apparatus having the advantages of effecting near-instantaneous power factor correction which is substantially continuous and precise for widely varying reactive loads, without substantial overcorrection, and of correcting harmonic disturbances and other undesirable high frequency resonances associated with a variable reactive load.